In order to prevent particulate matter (hereinafter, also referred to as PM) in gas discharged or emitted from an internal combustion engine into the atmosphere, a filter for trapping particulate matter may be provided in an exhaust passage of the internal combustion engine. When a large amount of PM is trapped by this filter, there will be a fear that clogging may occur in the filter. On the other hand, by making high the temperature of the filter as well as the concentration of oxygen in the exhaust gas, the PM trapped by the filter can be oxidized, so that the PM can be removed from the filter. The removal of the PM from the filter in this manner is referred to as regeneration of the filter.
Then, there has been known a technology which enhances efficiency at the time of raising the temperature of a filter by inhibiting the regeneration of the filter when the temperature of ambient air is equal to or less than a predetermined value (for example, refer to a first patent literature).
In addition, there has been known a technology in which at the time of regeneration of a filter, NOx is supplied to the filter from an NOx storage reduction catalyst (hereinafter, referred to as an NSR catalyst) which is provided at the upstream side of the filter (for example, refer to a second patent literature). In this technology, oxidation of PM is promoted by NOx. Also, a necessary amount of NOx is caused to be adsorbed to the NSR catalyst in advance before the regeneration of the filter.
Moreover, there has been known a technology in which a three-way catalyst is provided at the downstream side of a filter, so that the air fuel ratio of an exhaust gas is adjusted to be in the vicinity of a stoichiometric air fuel ratio at which NOx is able to be removed or reduced in the three-way catalyst, after which the air fuel ratio is adjusted to be a lean air fuel ratio with which the filter is able to be regenerated (for example, refer to a third patent literature).
Further, there has been known a technology in which a gas of a rich air fuel ratio is supplied to an NSR catalyst in a continuous or intermittent manner until the temperature of the NSR catalyst becomes equal to or less than a threshold value after the regeneration of a filter (for example, refer to a fourth patent literature).
In addition, there has been known a technology in which an amount of PM discharged from an internal combustion engine is decreased by decreasing an amount of EGR gas at the time when an amount of PM trapped by a filter is equal to or less than a predetermined amount, or by decreasing the amount of EGR gas immediately after regeneration of the filter (for example, refer to a fifth patent literature).
However, a filter, an NSR catalyst and an NOx selective reduction catalyst (hereinafter, also referred to as an SCR catalyst) may be provided in an exhaust passage of an internal combustion engine. Here, note that the NSR catalyst serves to occlude or store NOx contained in an incoming exhaust gas when the concentration of oxygen in the incoming exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists. In addition, the SCR catalyst serves to carry out selective reduction of NOx by means of the reducing agent. Then, by causing the internal combustion engine to operate at a rich air fuel ratio for only a short period of time during the operation of the internal combustion engine at a lean air fuel ratio, NH3 can be made to generate in the NSR catalyst. The NH3 thus generated can be supplied to the SCR catalyst as the reducing agent. In addition, by causing the internal combustion engine to operate at a rich air fuel ratio, HC and CO can be supplied to the NSR catalyst as the reducing agent.
Here, the trapping efficiency of the filter drops immediately after carrying out the regeneration of the filter. This trapping efficiency of the filter is a ratio of an amount of PM trapped by the filter with respect to an amount of PM flowing into the filter. This amount of PM may also be an amount of PM per unit volume, or may also be a concentration of PM. When the amount of PM trapped in the filter is large, a passage in the filter for exhaust gas is narrowed due to the PM thus trapped, so that it is easy for PM to be trapped. On the other hand, by carrying out the regeneration of the filter, the passage in the filter for exhaust gas is enlarged, so that it becomes easy for PM to pass through the filter. In addition, when the internal combustion engine is operated at a rich air fuel ratio in order to supply the reducing agent to the NSR catalyst, the amount of PM discharged from the internal combustion engine will be increased. Accordingly, immediately after the regeneration of the filter, when the internal combustion engine is operated at a rich air fuel ratio, there is a fear that PM may flow out to the downstream side of the filter. However, in cases where a device for supplying a reducing agent into the exhaust passage of the internal combustion engine is not provided, unless the internal combustion engine is operated at a rich air fuel ratio, it becomes impossible to supply the reducing agent to the NSR catalyst and the SCR catalyst, thus making it difficult to reduce NOx.